Recuperative furnaces are well known in the art. They are also known as condensing furnaces. They are distinguished from conventional or non-condensing furnaces by their recovery of a portion of the latent, as well as sensible, heat of the water vapor formed in the combustion process.
Non-condensing furnaces exhaust flue or vent gases at temperatures up to 550.degree. F. As a result, such furnaces produce flue gases in which the water generated by the combustion processes remains in the gaseous state; the latent heat of vaporization is not recovered. The efficiency of a furnace which operates in a non condensing mode generally has a maximum in the range from 75 to 85 percent. On a seasonal basis, this efficiency is reduced because usable heat is also lost up the chimney through a draft hood during the cool-down period at the end of each heating cycle.
Recuperative furnaces, on the other hand, do not employ draft hoods. The flue products are cooled to the dew point while still in the appliance, and some of the latent heat of vaporization is recovered as usable energy. This results in substantially higher efficiencies as less energy is lost out the flue. Vent gas temperatures may be as low as 100.degree. F. and there are few, if any, off cycle losses. Accordingly, depending on the type of condensing or recuperative furnace, efficiencies can be in the low to high 90 percent range.
The condensate produced by a recuperative furnace has a typical pH range of two to six; it is mildly acidic and potentially corrosive. By contrast, normal household waste water tends to be slightly alkaline. Its pH runs on the high side of seven.
The primary difference between the environment in a condensing or recuperative heat exchanger furnace and a conventional furnace is the presence of liquid water. The various gases in the flue gas dissolve in this water to form such acids as carbonic, sulfurous, sulfuric, nitric and nitrous. Dissolved oxygen and carbon monoxide are also present. The acid gases lower the pH of the water and promote acid corrosive attack. The combination of carbon monoxide and carbon dioxide may produce stress corrosion attacking some materials of construction as well. Likewise, sulfurous acid has been shown to promote stress corrosion cracking and intergranular attack of some materials.
The liquid water and hence the area of corrosion in a condensing heat exchanger occurs beyond the point at which the temperature of the flue gas falls below the water dew point. The dew point varies with the amount of excess air. However, increasing the amount of excess air is not the solution to the problem since excess air increases stack losses, thereby reducing furnace efficiency.
The acid dew point of flue gas is the temperature at which the acid compound sulfuric acid will drop out as a liquid. This temperature which is on the order of 260.degree. F. is much higher than the water dew point. Sulfurous, nitric, nitrous and carbonic acids will only be found below the water dew point because they only exist in a water solution.
The corrosive effects of these various acids on the furnace will vary from furnace to furnace and from fuel to fuel. Localized attack is probably the major problem to contend with, either in the form of pitting or possibly intergranular attack. Attack at welds is particularly critical. Sensitization of austenitic materials by welding promotes both intergranular attack and stress corrosion cracking. Welded construction may also introduce crevices in which accelerated attack may occur. Cyclic stressing of condensing a heat exchanger may result in shortening of the fatigue life by the mechanism of corrosion fatigue. Galvanic corrosion is also a possibility if two or more alloys are connected in the wet zone of a heat exchanger or at the drain. Use of two different materials is usually accomplished by brazing in which case the resistance of the braze itself and also the galvanic action must be considered.
The rate of attack of these corrosion mechanisms is dependent upon the design and the construction of each furnace. The use of materials such as stainless steel and plastic-coated parts will reduce the effects of corrosion. The direction of flow of condensate is also important. A furnace, for example, where the flue gas is forced up and out of the furnace will create a situation of condensation, run-down, and re-evaporation which will result in retention and concentration of sulfuric acid and high corrosion rates.